Molecular Mechanisms of Programmed Cell Death During Somatic Embryo Detachment from Callus in Indirect Somatic Embryogenesis.

Somatic embryogenesis in plants occurs via direct or indirect routes. In indirect somatic embryogenesis, embryos initiate from callus. The detachment of these embryos from callus is a critical step for plantlet recovery. Most reviews focus only on embryo induction. This review focuses specifically on the detachment stage. We examine how programmed cell death (PCD) drives this physical release. Cell death occurs in boundary callus cells and the suspensor. It does not destroy the developing embryo proper. PCD performs three main tasks. It dissolves middle lamella pectins. It removes the temporary suspensor. And it digests doomed cells to recycle nutrients for the embryo. Key proteases such as metacaspases execute this death program, with vacuolar processing enzymes representing candidate execution proteases. Epigenetic marks and transcription factors restrict cell death to the boundary. We also discuss practical ways to regulate PCD in tissue culture.